1. Field of the Invention
The present invention relates to a fluorescent lamp, a bulb-shaped fluorescent lamp, and a lighting apparatus having a fluorescent lamp or a bulb-shaped fluorescent lamp.
2. Description of the Related Art
In recent years, lighting apparatuses having a fluorescent lamp have become smaller, and their output has increased. In small and high-output lighting apparatuses, however, the light output of the fluorescent lamp tends to decrease. The smaller the lighting apparatus and the greater its output, the higher the temperature in the light-emitting tube of the fluorescent lamp becomes, with the result that the mercury-vapor pressure in the light-emitting tube is likely to increase. In order to suppress the excessive rise of the mercury-vapor pressure, fluorescent lamps for use in places where their intra-tube temperature may rise have a light-emitting tube filled with a main amalgam.
In the fluorescent lamp provided with the main amalgam, its light-emitting efficiency increases because the main amalgam suppresses an excessive rise of mercury-vapor pressure, as described above. However, a long time is required after a fluorescent lamp of this type is turned on, until the lamp starts emitting a predetermined luminous flux, i.e., the fluorescent exhibits a poor flux-startup characteristic. This is because the main amalgam suppresses the mercury-vapor pressure not only while the lamp is turned on, but also while the intra-tube temperature is as low as room temperature as occurring before the lamp is turned on, as compared to the fluorescent lamps filled with pure mercury. The fluorescent lamp having main amalgam emits a weak luminous flux immediately after it is turned on, due to the insufficient mercury-vapor pressure, though the luminous flux gradually increases as the intra-tube temperature rises, raising the mercury-vapor pressure in the sealed glass tube.
For these reasons, the fluorescent lamp having the main amalgam is provided with an auxiliary amalgam at a portion near the electrode, where the temperature can readily rise when the lamp is turned on. This adds a pressure to the mercury-vapor pressure in the light-emitting tube immediately after the lamp is turned on, thereby improving the flux-startup characteristics.
As auxiliary amalgam with which fluorescent lamps are provided, one is known that comprises a base made of stainless steel on which indium (In) is plated. However, this auxiliary amalgam is high in adsorption power for mercury, lowering the mercury-vapor pressure even more, while the lamp is turned off.
Further, as an auxiliary amalgam with which fluorescent lamps are provided, one is known which comprises a base on which gold (Au) is plated, as disclosed in Jpn. Pat. Appln. KOKAI Publication 2001-84956. Gold does not adsorb mercury excessively while the lamp remains off, and thus can maintain the mercury-vapor pressure relatively high at room temperature. It follows that a fluorescent lamp with auxiliary amalgam that comprises a base on which gold is plated can attain a large output immediately after it is turned on. Gold has a high melting point and hardly evaporates, and is hardly oxidized in the heating step during the manufacture of the fluorescent lamp. In view of this, gold is desirable for providing an auxiliary amalgam.
In the fluorescent lamp disclosed in Jpn. Pat. Appln. KOKAI Publication 2001-84956, however, the auxiliary amalgam has but a short lifetime. That is, the lamp obtains only a short period of time during which a flux-startup characteristic is improved. This is because gold is likely to diffuse into the base made of stainless steel (solid phase diffusion). Note that the gold layer plated on the stainless-steel base makes up for the mercury-vapor pressure in the fluorescent lamp immediately after the lamp is turned on. The gold therefore diffuses into the stainless-steel base. When the gold on the base decreases in amount, the auxiliary amalgam can no longer serve to provide a good flux-startup characteristic.
The technique described in Jpn. Pat. Appln. KOKAI Publication 2001-84956 may be employed to maintain a good flux-startup characteristic for a long time. In this case, the auxiliary amalgam must be plated with a thick gold layer. Gold is very expensive material. The thicker the gold layer, the higher the manufacturing cost of the fluorescent lamp.